1. Field of the Invention
The present invention relates to a control system for an internal combustion engine having an exhaust system provided with an HC adsorbent for adsorbing hydrocarbon, and more particularly to a control system for controlling an intake air amount, ignition timing, and/or exhaust gas recirculation amount.
2. Description of the Related Art
In an internal combustion engine having an exhaust system provided with a three-way catalyst and an HC adsorbent for adsorbing hydrocarbon (HC) contained in exhaust gases at cold starting of the engine, it is known that the HC is adsorbed by the HC adsorbent until the three-way catalyst is activated, thereby reducing the amount of HC emitted at cold starting of the engine. When the temperature of the HC adsorbent becomes high, the HC adsorbed by the HC adsorbent is desorbed. Accordingly, in the case that the HC adsorbent is provided downstream of the three-way catalyst, there is a problem of how to process the HC desorbed from the HC adsorbent. As a technique for dealing with this problem, it is known that the HC desorbed from the HC adsorbent is recirculated to an intake system of the engine (Japanese Patent Laid-open No. 10-153112).
However, the exhaust gas recirculation is not performed during a low-load operation of the engine, including an idling operation. Accordingly, in the case that the engine is started and only the idling operation or very-low-speed running of the vehicle driven by the engine is subsequently performed until the engine is stopped, the exhaust gas recirculation is not performed. As a result, the HC adsorbed by the HC adsorbent is not completely desorbed, and newly introduced HC cannot be sufficiently adsorbed by the HC adsorbent at the next starting of the engine.
It is accordingly an object of the present invention to provide a control system for an internal combustion engine which can reliably discharge HC adsorbed by an HC adsorbent to maintain exhaust emission characteristics at starting of the engine in a good condition.
The present invention provides a control system for an internal combustion engine having an exhaust system provided with an adsorbent for adsorbing hydrocarbon. The control system includes adsorbent temperature raising means for increasing an intake air amount of the engine and retarding an ignition timing of the engine when the adsorbent has adsorbed the hydrocarbon and the engine is operating in an idling condition.
With this configuration, when the adsorbent has adsorbed the hydrocarbon and the engine is operating in an idling condition, the intake air amount of the engine is increased and the ignition timing of the engine is retarded. Accordingly, the rise in the exhaust gas temperature can be accelerated to thereby accelerate the rise in temperature of the adsorbent. As a result, the time period required for completion of the desorption of the hydrocarbon adsorbed by the adsorbent can be shortened, and it is therefore possible to reduce the number of cases in which the engine is stopped in the condition where the desorption is incomplete, thereby enhancing the exhaust gas purification effect by the adsorbent.
Preferably, the adsorbent temperature raising means gradually increases the intake air amount and gradually retard the ignition timing.
With this configuration, the intake air amount is gradually increased and the ignition timing is gradually retarded. Accordingly, stable combustion can be continued even under the conditions where the combustion is prone to become unstable, such as immediately after starting of the engine at a low ambient temperature or at a low atmospheric pressure.
Preferably, the adsorbent temperature raising means retards the ignition timing so as to maintain a rotational speed of the engine at a target rotational speed.
With this configuration, the ignition timing is retarded so that the engine rotational speed is maintained at the target rotational speed. Accordingly, the ignition timing can be automatically set to an optimum ignition timing which changes according to an ambient temperature, atmospheric pressure, or engine operating conditions.
Preferably, the exhaust system includes a catalyst for exhaust gas purification, and the hydrocarbon desorbed from the adsorbent is recirculated to a portion upstream of the catalyst.
With this configuration, the hydrocarbon desorbed from the adsorbent is recirculated to the upstream side of the catalyst, so that the hydrocarbon adsorbed by the adsorbent can be reliably purified (oxidized).
Preferably, the adsorbent temperature raising means decreases the intake air amount when the ignition timing is retarded beyond a predetermined retard limit value.
With this configuration, when the ignition timing is retarded beyond the predetermined retard limit value, the intake air amount is decreased. Accordingly, it is possible to prevent an increase in engine rotation fluctuation and a deterioration of the engine combustion due to an excessive retard of the ignition timing.
The present invention further provides a control system for an internal combustion engine having an exhaust system, an intake system, and an exhaust gas recirculation mechanism for recirculating exhaust gases from the exhaust system to the intake system. The exhaust system is provided with an adsorbent for adsorbing hydrocarbon. The control system includes recirculation control means for performing recirculation of the exhaust gases through the exhaust gas recirculation mechanism to recirculate the hydrocarbon desorbed from the adsorbent to the intake system when the adsorbent has adsorbed the hydrocarbon and the engine is operating in an idling condition.
With this configuration, when the adsorbent has adsorbed the hydrocarbon and the engine is operating in an idling condition, the exhaust gas recirculation is performed to recirculate the hydrocarbon desorbed from the adsorbent to the intake system. Accordingly, even when only the idling operation is carried out after starting of the engine until the engine is stopped, the hydrocarbon adsorbed by the adsorbent can be reliably desorbed to thereby maintain good exhaust emission characteristics immediately after starting of the engine.
Preferably, the control system further includes intake air amount increasing means for increasing an intake air amount of the engine during execution of the exhaust gas recirculation by the recirculation control means.
With this configuration, the intake air amount of the engine is increased during execution of the exhaust gas recirculation. Accordingly, it is possible to prevent instability of the combustion due to the exhaust gas recirculation during idling of the engine, to thereby maintain a stable idling operation.
Preferably, the intake air amount increasing means controls the intake air amount so that an increase in the intake air amount becomes greater than a decrease in the intake air amount due to the exhaust gas recirculation.
With this configuration, the increase in the intake air amount is controlled to become greater than the decrease in the intake air amount due to the exhaust gas recirculation. Accordingly, the thermal energy of the exhaust gases can be made larger than that in a normal idling operation, so that the temperature rise of the adsorbent is accelerated to shorten the time period required for completion of the desorption of hydrocarbon.
Preferably, the control system further includes ignition timing correcting means for correcting an ignition timing of the engine when the intake air amount is increased during execution of the exhaust gas recirculation by the recirculation control means.
With this configuration, the ignition timing is corrected when increasing the intake air amount of the engine during execution of the exhaust gas recirculation. Accordingly, by correcting the ignition timing to an optimum value, instability of the combustion can be prevented.
Preferably, the amount of the recirculated exhaust gases, the intake air amount, and the ignition timing are gradually changed.
With this configuration, stable combustion can be continued even under the conditions where the combustion is prone to become unstable, such as immediately after starting of the engine at a low ambient temperature or at a low atmospheric pressure.
Preferably, the ignition timing correcting means corrects the ignition timing so as to maintain a rotational speed of the engine at a target rotational speed.
With this configuration, the ignition timing is corrected so that the engine rotational speed is maintained at the target rotational speed. An optimum ignition timing changes with the amount of hydrocarbon mixed into the intake air, because the hydrocarbon desorbed from the adsorbent is recirculated to the intake system. However, by setting the ignition timing so as to maintain the engine rotational speed at the target speed, the ignition timing can be automatically set to an optimum ignition timing.
Preferably, the recirculation control means increases the amount of the recirculated exhaust gases with a delay or a dead time with respect to the increase of the intake air amount and the correction of the ignition timing.
With this configuration, the increase of the exhaust gas recirculation amount is performed with a delay or a dead time with respect to the increase of the intake air amount and the correction of the ignition timing. Accordingly, engine rotation fluctuation at starting of the exhaust gas recirculation can be minimized.
Preferably, the ignition timing correcting means corrects the ignition timing after the increase of the intake air amount.
With this configuration, the ignition timing is corrected after increasing the intake air amount. Accordingly, it is possible to prevent instability of the combustion when increasing the intake air amount.
Preferably, the intake air amount increasing means decreases the intake air amount when the ignition timing is retarded beyond a predetermined retard limit value.
With this configuration, when the ignition timing is retarded beyond the predetermined retard limit value, the intake air amount is decreased. Accordingly, it is possible to prevent an increase in engine rotation fluctuation and a deterioration of the engine combustion due to an excessive retard of the ignition timing.
Preferably, the control system further includes rotational speed fluctuation amount detecting means for detecting a fluctuation amount of rotational speed of the engine. The recirculation control means increases the amount of the recirculated exhaust gases when the fluctuation amount detected by the rotational speed fluctuation amount detecting means is less than a first predetermined fluctuation amount.
With this configuration, when the detected engine rotation fluctuation amount is less than the first predetermined fluctuation amount, the exhaust gas recirculation amount is corrected to increase. Accordingly, the amount of hydrocarbon desorbed from the adsorbent and recirculated to the intake system can be increased up to such a degree that the engine rotation does not become unstable, thereby shortening the time period required for completion of the desorption of hydrocarbon.
Preferably, the recirculation control means decreases the amount of the recirculated exhaust gases when the fluctuation amount detected by the rotational speed fluctuation amount detecting means is greater than a second predetermined fluctuation amount.
With this configuration, when the detected engine rotation fluctuation amount is greater than the second predetermined fluctuation amount, the exhaust gas recirculation amount is corrected to decrease. Accordingly, it is possible to prevent instability of the engine rotation due to an excessive exhaust gas recirculation.
Preferably, the recirculation control means decreases the amount of the recirculated exhaust gases when the ignition timing is advanced beyond a predetermined advance limit value.
With this configuration, when the ignition timing is advanced beyond the predetermined advance limit value, the exhaust gas recirculation amount is corrected to decrease. Accordingly, it is possible to avoid such a situation that an advance control range of the ignition timing may be reduced to cause instability of the engine rotational speed control.